1. Field of the Invention
The present invention relates to an apparatus and electrochemical method for preparing peracids.
2. Description of the Related Art
Peracids have long been used as chemical reagents and bleaching agents in industrial chemical processes. Some peracids are effective biocides and so are often used as microbial and scale inhibitors in water treatment systems. Increasingly, dilute aqueous solutions of peracids are being used as disinfectants in environmental, agricultural, medical, and food processing applications. In 1986, peracetic acid received FDA approval for use as a clean-in-place (“CIP”) oxidizer in the food and beverage industry. More recently, peracetic acid products have received FDA approval for direct use on fruits and vegetables
A peracid is the oxidized derivative of a carboxylic acid, for example, peracetic acid (CH3COOOH) is the oxidized derivative of acetic acid (CH3COOH). Peracids are strong oxidizing agents and most (e.g., per acetic acid, per lactic acid, etc.) are completely soluble in water. Thus, peracid solutions provide an alternative to chlorine, ozone or hydrogen peroxide solutions, especially for applications where off-gassing is a concern. Peracids are germicidal and readily decompose into non-toxic constituents. However, peracids have been found to be sporicidal and remain effective at dilute concentrations even in the presence of organic matter. In fact, cooling tower studies show that peracetic acid remains effective at concentrations as low as 3 to 5 ppm.
Some advantages of using aqueous peracid solutions for sterilizing and disinfecting applications are attributable to the fact that peracids are: (1) sporicidal, bactericidal, virucidal and fungicidal at dilute concentrations (5-35 ppm) in water; (2) effective against non-tuberculous mycobacteria including chelonae, HIV, hepatitis B viruses, micro-organisms, spores, viruses and fungi; (3) capable of denaturing proteins and enzymes; (4) soluble in lipids and so is not deactivated by microbial enzymes like catalase and peroxidases; (5) rapidly decomposed into the parent carboxylic acid, water and oxygen without leaving toxic residues; and (6) safe for use in sterilizing medical instruments and equipment, disinfecting food preparation surfaces and cleaning floors, walls, plumbing, dishes, and toys.
Antimicrobial compositions are particularly needed in the food and beverage industries to clean and sanitize processing facilities such as pipelines, tanks, mixers, etc. and continuously operating homogenization or pasteurization equipment. Other uses for antimicrobial compositions include vegetable washing and disinfection, meat surface decontamination, poultry chiller baths, processing equipment, cleaning and disinfecting beverage containers, sterilizing point-of-use and point-of-entry water purification devices, terminal sterilization and treatment of contaminated infectious waste. The biocidal activity of peracids results from its oxidation of sulfhydryl groups (—SH), disulfide bonds (S—S) and double bonds in proteins, lipids and other cellular constitutents to disrupt the chemiosmotic functions of the cell membrane. This broad oxidizing capability controls biofilm and scale deposits by eliminating the accumulation of biomass, mold, mildew, algae, fungi and bacteria on numerous surfaces and in aqueous systems in general.
The presence of disease causing microorganisms, disinfection byproducts and other toxic substances in drinking water poses a great health risk to humans. Point of use activated carbon filters and reverse osmosis (“RO”) units are becoming increasing popular with consumers for water purification. These water purification devices are primarily used for removing municipal disinfectants, i.e., chlorine, and metals from tap water. However, if bactericides are not used, the filtration media and membranes quickly become a breeding ground for heterotrophic (HTPC) bacteria.
Because some peracids have a high oxidation potential, these same peracids are excellent biocides even at dilute concentrations. In contrast, hydrogen peroxide needs 100 times greater concentrations to achieve comparable biocidal activity. However, at these higher concentrations, the off-gassing of hydrogen peroxide can become a limiting factor. Furthermore, hydrogen peroxide does not retain its anti-microbial activity in the presence of interfering compounds, because it reacts indiscriminately with dissolved oxidizable substances such that the amount of hydrogen peroxide available for disinfection is drastically reduced. However, when used in solution with peracids, lower concentrations of hydrogen peroxide can be used. In fact, mixtures of hydrogen peroxide and peracids can enhance the disinfecting and sterilizing of a variety of materials including surgical and medical devices.
In most applications, it is preferable to use dilute aqueous solutions of peracids (ppm level) for sterilizing or disinfecting. However, because peracids are so highly reactive, it is relatively costly to synthesize, transport and deliver peracids as oxidizing agents using conventional processes. For example, a 1% solution of peracetic acid loses half its strength through hydrolysis in six days. In this regard, peracetic acid is less stable than hydrogen peroxide and becomes increasingly less stable when diluted. For this reason, stabilizers are often added to undiluted peracid solutions which must then be diluted prior to use. Clearly a more efficient process for generating peracids at the “point of use” would greatly increase their use by the general public.
Peracids are typically produced by chemical synthesis as a result of mixing carboxylic acid and hydrogen peroxide with an inorganic acid catalyst in a reaction vessel. For example, peracetic acid is commercially produced by the reaction of hydrogen peroxide with acetic acid (or acetic anhydride) using concentrated sulfuric acid as a catalyst. Because the peracids generated are highly reactive, stabilizers are added to maintain the oxidizing capacity of peracids during storage and shipment. Furthermore, because many of these additives and other process by-products are toxic or hazardous, they entail more costly storage, handling and transportation costs. These additives are retained in the peracid product and generally require further treatment prior to use or disposal. Clearly a process capable of generating high purity solutions of peracids would be beneficial.
Peracids can also be produced in electrochemical cells wherein the desired reactions are carried out by imposing an electric current across electrodes immersed in an electrically-conducting fluid, such as water. In an electrochemical cell, the liquid acid catalyst is replaced by, solid electrolyte, such as a perfluoronated sulfonic acid polymer, thus eliminating the need for corrosive acids.
U.S. Pat. No. 5,122,538 (Lokkesmoe et al.) discloses a process for generating peracid in a packed-bed type reaction vessel charged with a cation-exchange resin (e.g., sulfonic acid resin). The resin catalyzes the reaction between the hydrogen peroxide and a carboxylic acid to produce peracids. One problem with this method is that the hydrogen peroxide causes the resins to swell. To reduce this swelling, the catalyst bed must be regenerated with chelating agents prior to introducing the reaction mixture. Common chelating agents include ethylene-diaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA) and organic phosphates, such as phytic acid. The resins may degrade in the presence of oxidizing agents which can introduce contaminants into the peracid product and limit catalyst life in general. In addition, some of the chelating agents and reaction products present in the resulting product are toxic. Thus, health and safety concerns limit or prevent the direct application of peracids generated by this process for medical sterilization, food processing and consumer product applications.
U.S. Pat. No. 6,171,551 B1 (Malchesky et al.) discloses a method for generating peracid at the anode of an electrochemical cell. The process generates the oxidizing species, e.g., a mixture of ozone and hydrogen peroxide in the anode compartment. These oxidizing species are then reacted with an aqueous organic acid solution within the anode to produce a peracid solution in the anode compartment. After sufficient concentrations of the peracid are formed, the anode product containing peracid and the oxidizing species are withdrawn from the anode compartment. In addition, it is disclosed that a lag-time of between one and two hours is needed before any detectable levels of the oxidizing species are produced. This means that the method is not conducive to continuous operation, and highly inappropriate for use as a POU/POE device. Furthermore, the reported bench-scale results indicate that the quantities of hydrogen peroxide and peracetic acid produced are very low.
Existing electrochemical peracid generating processes suffer from many disadvantages including low production rates and the potential for introducing toxic constituents. In addition, in the preparation of peracids by the chemical synthetic route, the use of concentrated hydrogen peroxide (30-70%) as a raw material and also the use of concentrated sulfuric acid as a catalyst pose safety problems for transportation, storage and handling of the reactants and the catalyst. In fact, because peroxide solutions are unstable, inorganic stabilizers are generally added to prevent off-gassing of hydrogen peroxide. Further, additional stabilizers are used to stabilize peracids. Amongst those which have found wide application include dipiconilic acid, for example as disclosed in U.S. Pat. No. 2,609,391. Other compounds include phosphonates, notably those disclosed in British patent No. 925,373. In other instances, a combination of stabilizers are employed, for example the combinations of dipicolinic acid and phosphonates disclosed in International application publication Nos. WO91/07375 and WO91/13058. The use of stabilizers and other additives can introduce further toxic or otherwise hazardous compounds into the peracid solution.
Therefore, there exists a need for an apparatus and a method that can generate high purity peracid solutions without any toxic or otherwise undesirable by-products and thereby, promote their use in numerous disinfecting and sterilizing applications. There is also a need for a method and apparatus that produces peracids quickly and essentially on demand. It would be desirable if the method and apparatus would provide POU/POE generation of peracids.